Possible answers to the above questions:
1. Di belakang (At the back) or Di tingkat atas (On the top floor)
2. Di bawah meja (Under the table) or Dekat pintu (Near the door)
3. Di sebelah kiri (To the left) or Di sebelah kanan (To the right)Di sebelah here means "to the side of".
4. Di hadapan anda (In front of you) or Di sana (Over there)
5. Di kedai kopi (In the coffee shop) or Di pejabat (In the office)
6. Di atas meja (On the table) or Di dalam beg anda (In your bag)
But if you don't know the answer to any of the above questions you can simply say Saya tidak tahu or you can surprise your friend by using a very "Malay" expression meaning the same thing: Entahlah (entah meaning "don't know").
In case you have forgotten that the verb "to be" is often left out in Malay, you will be reminded that this is so from the questions above. So in Malay you only need to ask "Where washroom?, Where my shoes?, Where Petaling Street?, Where he?, Where my key?" Don't laugh - after all Malay is not English!
While di can mean "in" or "at" as in the sentence Dia ada di rumah (He is at home) I would suggest that you study di mana to mean "where" (although mana alone also means "where"). If it can help you in any way di mana literally means "in where".
By the way you will notice that di is not joined to the word mana so as to become a single word. In fact di is only joined to verbs so that they become passive as you would have seen in earlier lessons (eg. Dia telah dipukul oleh dua orang samseng means "He was beaten up by two gangsters"). When used with adverbs, however it is not joined to them to become a single word but is detached from them eg. di atas (= above), di bawah (= below), di sana (= over there) and not diatas, dibawah, disana. Not that it is that important, as some Malaysians themselves do not get it right, but good to know.
Note:
You might want to replace bilik air with tandas (= toilet) or with a much less exquisite word jamban. Any of these three words would be understood in Malaysia for the washroom or toilet though in some inward areas of the country jamban could be just the place where you defecate and may not even have a washbasin. So better stick to tandas or bilik air if you can. Incidentally while "bathroom" can also mean "washroom" in American English, in Malaysia it can only mean the place where one goes to take one's bath, in which case the word is bilik mandi (mandi meaning "to bathe"). So only if you intend to go for your bath should you ask Mana bilik mandi?Okay, since we are on this topic let me introduce the two relevant words (apart from the obvious cuci tangan = wash hands). The two words are:
kencing meaning "to urinate" and
berak (the first syllable is pronounced as "bay") meaning "to poo, defecate".
Or if you prefer to use a more refined language, you can also say buang air kecil for the first and buang air besar for the second. If you are curious to know what they mean well the first means "throw little water" and the second "throw big water" - very illuminating indeed! Incidentally this concept of "little" and "big" is also used in Mandarin where to urinate is siao3 bian4 and to defecate is da4 bian4, siao3 meaning "little" or "small" and da4 meaning "big". Wonder if other languages think alike or is it just Asians who think that way.

(September 14, 2015)
Nanie writes in to say:
In French we have the same kind of expression.
We say "Faire la petite commission" for urinate
and "Faire la grosse commission" for defecate
In French "commission" has a lot of different meanings but used in this context it would mean "the task" or "the thing".
"Petite" is feminine of small and "grosse" feminine of big , "faire" just means do
So you do the small or big thing...Thank you, Nanie! - Webmaster

And if you want more of Malaysian euphemisms (in English this time) instead of saying that you are going to "urinate" you can say that you are going to "put out the fire" while if you need to poop, you can say you're going to the bank! You know you are understood when the listener gives a chuckle instead of asking how much money you are going to withdraw! And also if you are still in your pyjamas and not dressed up for your banker!
Yet another example of euphemism in the Malay language is when you want to say that someone has died. The Malay word for "die" is mati. But the Malays find it too harsh a word to use so instead of saying Dia sudah mati (= He has died) they would often say Dia sudah meninggal dunia. (= He has left the world). By the way you might not be able to find the verb meninggal in a Malay dictionary. This is because it comes from the verb tinggal. Full explanation on the formation of Malay prefixes (a real nightmare for students!) in the Appendix.
By the way the old spelling of air (which in Malay does not mean the air that we breathe but the water that we drink!) was "ayer" which will give a better indication of its pronunciation (that is, as two syllables ah-ear).
But enough of all this digressing - let's get back to our lesson!

More questions:Di mana perhentian bas? (Where is the bus-stop?)Di mana lapangan terbang? (Where is the airport?)Di mana stesen kereta api? (Where is the railway station?)
Note that "station" is spelt stesen and not stesyen as it would normally be if pronounced the English way. (More on this in Lesson 49).

Cultural note:
When one approaches a stranger with a question one does not normally put the question abruptly to him. It is customary for one to start with a preliminary question: Boleh saya tumpang tanya? meaning "Do you mind if I ask you a question?" or Boleh saya tumpang tanya sedikit? (Can I ask you a bit?)As a stranger it is better to get the go-ahead before you ask the real question though Malaysians might just take the answer for granted and start asking the real question immediately after eg. Boleh saya tumpang tanya, apa nama jalan ini? meaning "Don't mind if I ask you the name of this road?"
Anyway in my whole life I have never got "No" for an answer to this question (the preliminary question I mean) as the Malays are without doubt a very friendly race (and all Malaysians too, for that matter).
It's best to learn this expression by itself without worrying too much about the meaning of the word tumpang. But if you are really curious to know what it means I won't hide it from you. It is normally used in the following two cases:
In the sense of making use of someone's facilities. Thus if you are putting up at a friend's place you would say you tumpang at his place. This is quite a common practice among Malaysians who when travelling usually put up at their friends' or their relatives' house instead of staying in hotels. It's not so much that they are trying to save money on hotel charges (the gifts that they bring along would have paid for the hotel room!) but it's just part of their cultural heritage, that's all (this is slowly changing with the times though).
Sorry, I've digressed a bit. To come back to our subject. The word tumpang is also used when you are asking for a lift in a friend's car eg. Boleh saya tumpang kereta anda ke bandar? (Can I get a lift in your car to the city?)

Mnemonics:
It's so easy to confuse the words for "left" and "right" more so when they are learnt at the same time. From the pedagogical point of view only one of these two words should be introduced in class until it sinks in, then only the other in order to avoid confusion. Just imagine turning left when you are told to turn right just because you mixed up the two words. Scary, isn't it?
However since this is not so practical in an online course, if you need to make use of mnemonics to help you not to mix them up I have one for you. Look at the two words and their English equivalents. The Malay word for "left" is kiri while the word for "right" is kanan. What do you notice? Firstly, that both words in Malay start with the letter "k" (not much of a help - only adds to the confusion). And secondly that whether it is in English or in Malay "left" has got four letters and "right" five. Now you will never ever mix them up again, will you?
A quick check before I say goodbye for today, is kiri right or left?